The Waltham single track section is less than a mile long and seldom causes delays of more than a minute or two. The eight miles between South Acton and Willows were formerly double track (until the 1960s) so unless there's been right-of-way encroachment there should be room to restore the second main.

I assume the notch right before S. Acton going outbound, where it intially turns single track, will be a bottleneck - I'm not sure there's room for two there, but I've only seen it from inside the train.

Part of the pain of the single track is that S. Acton is also a terminus for several of the trains - thus blocking all traffic while waiting for turnaround, lmiting the schedule. There is no outbound train between 1:20pm (or therabouts) and 4:40pm that goes past S. Acton.

They'd probably have to rebuild the platform at Littleton again to push it back, or finally move it like they've been talking about.

I did take a late outbound train that had to halt at Waltham due to a signal - for quite a while. Apparently the controller forgot about us...

First Law of Public Transportation: You can never be early, but you can always be late.

I imagine some of the money in the bond would go to Build the Littleton/495 station that keeps getting talked about. When the last itteration of that plan was being pushed, the design called for extending the double track from South Acton to the new station. The South Acton turn would also have been pushed out to Littleton.

That Waltham section was double track until about 1960. Single track and centered to accomodate autoracks (B & M carried American Motors cars into Boston for a few yrs. to KAT Yard - Kenosha Auto Transport.) Piggybacks had already run through there for a few yrs. and they were growing larger too.
The South Acton-Willows section was single tracked about 1965-66 after the big reduction in service west of South Acton. There was a lengthy middle siding, 110 cars or so, South Acton-West Acton that ran through both the Martin and Central Street crossings.
I seem to think a consultant has already done some very preliminary engineering regarding restoration of a second main track west of South Acton. My friends in railroad engineering departments have told me that restoring a double track is fairly easy. Engineering a double on a line built as single is much more difficul.